I run a web service that gets around 10m hits a month, and I serve one popup ad (yuk, I know) per day (or until cookies are cleared).

Compared to revenue, hosting cost is next to nothing for sites like Tumblr that serve text/image content. A similar video site would need to spend a ton of money on bandwidth, streaming CDNs, encoding servers, etc.

If you get 2.5 million hits a month, it is certainly possible to cover the said selling price of $3m in ads alone in a month.